The Internet Engineering Task Force (IETF) Request for Comments (RFC) document “OSPF Version 2,” RFC 2328, which is incorporated herein by reference, describes Open Shortest Path First (OSPF) areas in an Autonomous System (AS). Each area has a number of area border routers connected to the backbone. Each area border router summarizes the topology of its attached non-backbone areas for transmission on the backbone, and hence to all other area border routers. An area border router then has complete topological information concerning the backbone, and the area summaries from each of the other area border routers. From this information, the router calculates paths to all the destinations outside of its attached area. The router then advertises these paths into its attached areas. This enables the area's internal routers to pick the best exit router when forwarding traffic to destinations outside of the area.
The IETF RFC document “OSI IS-IS Intra-domain Routing Protocol,” RFC 1142, which is a republication of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) 10589, which are each incorporated herein by reference, describes Intermediate System to Intermediate System (IS-IS) areas in an AS, which are similar to OSPF areas. As used herein, OSI refers to an Open Systems Interconnection model.
Any link state change within an area may lead to a number of link state distributions to other areas. This will trigger routers in other areas to re-calculate their OSPF or IS-IS routes, and update their Routing Information Base (RIB) and Forwarding Information Base (FIB). All these will consume network resources including network bandwidth and Central Process Unit (CPU) time.
The topology transparent zone (TTZ) was recently proposed to resolve all these issues. However, routing interruptions may be experienced when a group of routers and a number of links connecting these routers are being transferred to a virtual entity such as the TTZ.